When voltage is applied on an organic electroluminescence device (hereinafter, occasionally referred to as an organic EL device), holes and electrons are respectively injected into an emitting layer from an anode and a cathode. The injected electrons and holes are recombined in an emitting layer to form excitons. Here, according to the electron spin statistics theory, singlet excitons and triplet excitons are generated at a ratio of 25%:75%. In the classification according to the emission principle, in a fluorescent EL device which uses an emission caused by singlet excitons, the limited value of an internal quantum efficiency of the organic EL device is believed to be 25%. On the other hand, in a phosphorescent EL device which uses an emission caused by triplet excitons, it has been known that the internal quantum efficiency can be improved up to 100% when intersystem crossing efficiently occurs from the singlet excitons.
A technology for extending a lifetime of a fluorescent organic EL device has recently been improved and applied to a full-color display of a mobile phone, TV and the like. However, an efficiency of a fluorescent EL device is required to be improved.
Based on such a background, a highly efficient fluorescent organic EL device using delayed fluorescence has been proposed and developed. For instance, Patent Literatures 1 and 2 disclose an organic EL device using TTF (Triplet-Triplet Fusion) mechanism that is one of mechanisms for delayed fluorescence. The TTF mechanism utilizes a phenomenon in which singlet excitons are generated by collision between two triplet excitons.
By using delayed fluorescence by the TTF mechanism, it is considered that an internal quantum efficiency can be theoretically raised up to 40% even in a fluorescent emission. However, as compared with a phosphorescent emission, the fluorescent emission is still problematic on improving efficiency. Accordingly, in order to enhance the internal quantum efficiency, an organic EL device using another delayed fluorescence mechanism has been studied.
For instance, TADF (Thermally Activated Delayed Fluorescence) mechanism is used. The TADF mechanism utilizes a phenomenon in which inverse intersystem crossing from triplet excitons to singlet excitons is generated by using a material having a small energy gap (ΔST) between the singlet level and the triplet level. An organic EL device using the TADF mechanism is disclosed in, for instance, non-Patent Literature 1. In the organic EL device of non-Patent Literature 1, a compound having a small ΔST is used as a dopant material to cause inverse intersystem crossing from the triplet level to the singlet level by heat energy. It is considered that the internal quantum efficiency can be theoretically raised up to 100% even in a fluorescent emission by using delayed fluorescence by the TADF mechanism,
Non-Patent Literature 2 discloses an organic EL device having a doped layer in which a specific host material and a specific compound having a spiro skeleton as a dopant material are used. The organic EL device exhibits a high external quantum efficiency with the use of the TADF mechanism.
Non-Patent Literature 3 discloses an organic EL device having a doped layer in which a specific host material and a compound having a triazine derivative skeleton as a dopant material are used. The organic EL device exhibits a high external quantum efficiency with the use of the TADF mechanism.